Surface resonator for nuclear magnetic resonance equipment

ABSTRACT

A surface resonator suitable for use in nuclear magnetic resonance equipment has two adjacently disposed windings, the respective ends of each winding being connected to each other through a series circuit including two capacitors. The respective junctions of the capacitors with each other constitute the electrical symmetry point for each winding, and are connected to ground. The signal tap for the surface resonator is galvanically taken at the ends of each winding. The surface resonator can be used for imaging and spectroscopy using nuclear magnetic resonance principles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a surface resonator having twoadjacently disposed windings of the type used in conducting examinationswith nuclear magnetic resonance equipment.

1. Description of the Prior Art

A surface coil of the type suitable for use in nuclear magneticresonance equipment having two adjacently disposed windings is describedin the article "Counter-Rotating Current Local Coils for High ResolutionMagnetic Resonance Imaging," in Magnetic Resonance in Medicine, Vol. 3,1986 at pages 590-603. Two loop-gap resonators are disclosed thereinwhich are disposed adjacent to each other, and electrically coupled toeach other. Two resonance conditions having different resonantfrequencies are possible, whereby the currents in the loop-gapresonators flow in the same direction in one case, and in oppositedirections in the other case. Coupling of this surface resonator to thenuclear magnetic resonance equipment is undertaken with a coupling coilarranged in the inside of the surface resonator.

Surface resonators are used in the same manner as surface coils whensignals from a locally limited examination region are to be acquiredwith increased resolution. Such surface resonators can also be used as atransmitter as well as a receiver for imaging and spectroscopy. In allapplications, however, the coupling coil disposed inside the surfaceresonator prevents complete symmetry from being achieved, so that therespective transmission and reception fields lose homogeneity.Transformation losses are also associated with the inductive coupling.When such a resonator is used for imaging, image disturbances occur as aresult of the presence of the coupling coil.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surface resonatorhaving two adjacently disposed windings which exhibits electricalsymmetry, and which therefore generates and receives homogeneous fields.

The above object is achieved in accordance with the principles of thepresent invention in a surface resonator having two adjacent windings,each winding having two free ends which are connected to each other viaa series circuit consisting of two capacitors. The junction pointbetween the capacitors is located at the electrical symmetry point forthe associated winding, which as used herein means the electricalcharacteristics (capacitance, impedance) on both sides of the electricalsymmetry point are equal. Signal input and signal output for eachwinding is also undertaken at this point, which is connected to amatching circuit for this purpose.

Each winding may simply consist of a ribbon or band of copper foil.

In one embodiment, the windings in the form of copper foil ribbons maybe mounted on a carrier in the form of a bar having a rectangular crosssection, the bar being shaped into a ring. The two windings are disposedon opposite sides of the ring. In a further variation of thisembodiment, another set of windings, with associated capacitors asdescribed above, can be disposed on the other two opposite sides of thering. The values of the capacitors for this second set of windings maybe selected different from the values for the first set of windings, sothat a separate resonator operable at a different frequency is formed.

The surface resonator described herein can be used at different resonantfrequencies for examining different nuclei by alternatively using one orthe other sets of windings.

A uniform sensitivity field for the surface resonator described hereinis obtained in a further embodiment wherein two surface resonators, asdescribed above, are disposed in a Helmholtz arrangement, with all ofthe axes for the respective windings disposed on a line.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a nuclear magnetic resonanceapparatus of the type in which the surface resonator disclosed hereinmay be used.

FIG. 2 is an electrical circuit diagram of a surface resonatorconstructed in accordance with the principles of the present invention.

FIG. 3 shows the mechanical arrangement of a surface resonatorconstructed in accordance with the principles of the present invention.

FIG. 4 is a sectional view taken along line IV--IV of FIG. 3 showing afurther embodiment of a surface resonator constructed in accordance withthe principles the present invention.

FIG. 5 is a side view of two surface resonators constructed inaccordance with the principles of the present invention in a Helmholtzarrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic components of a nuclear magnetic resonance device foridentifying the spectra or generating an image of an examination subjectare shown in FIG. 1. The apparatus includes coils 1, 2, 3 and 4 whichgenerate a fundamental magnetic field in which a patient 5 to beexamined is disposed. A plurality of sets of gradient coils are alsoprovided, which generate independent orthogonal magnetic field gradientsin the x, y and z directions of a coordinate system generally referencedat 6. For clarity, only gradient coils 7 and 8 are shown in the drawing.The coils 7 and 8, in combination with a pair of identical gradientcoils disposed on the opposite side of the patient 5, generate agradient field in the x-direction. Identical gradient coils (not shown)for generating a gradient field in the y-direction are disposed aboveand below the patient 5, parallel thereto. Coils for generating agradient field in the z-direction (not shown) are disposed transverselyrelative to the longitudinal axis of the patient 5, at the feet and headof the patient 5.

The apparatus also includes an antenna in the form of a body resonator 9for generating nuclear magnetic resonance signals in the patient 5.Surface resonators 19 and 19' are provided above and below the patient 5for receiving the nuclear magnetic resonances signals.

The coils 1, 2, 3, 4, 7, 8 and 9, surrounded by the dot-dash line 10,constitute the actual examination instrument in which the patient 5 isdisposed. This instrument is operated from an electrical arrangementwhich includes a power supply 11 for the fundamental field coils 1through 4, and a gradient power supply 12 for the gradient coils 7 and8, as well as the other gradient coils which are not shown in thedrawing.

A radio frequency transmitter 14, controlled by a process computer 17,is connected to the whole-body resonator 9. The surface resonators 19and 19' are also coupled to the computer 17 via a signal amplifier 15.The transmitter 14 and the signal amplifier 15 are components of aconventional radio frequency unit 16 for signal generation andreception.

The process computer 17, in a known manner, constructs a graphicalrepresentation of a spectrum, or an image, of the region of interest ofthe patient 5 from the incoming signals, and this image is visuallyrepresented on a display 18.

Details of the surface resonator 19 (to which the surface resonator 19'is identical) constructed in accordance with the principles of thepresent invention are shown in FIG. 2. The surface resonator 19 includestwo windings 19a and 19b disposed adjacent to each other. The ends ofeach winding 19a and 19b are connected to each other by a series circuitconsisting of two capacitors. The ends of winding 19a are connected viacapacitors 19c and 19d, and the ends of winding 19b are connected bycapacitors 19e and 19f. The center taps or junctions between thecapacitors 19c and 19d, and between the capacitors 19e and 19f,represent the electrical symmetry point for the winding respectivelyconnected thereto. Those points are connected to a grounded potential ina matching circuit 20. Signal input and output for the matching circuit20 is undertaken via a coaxial line 25.

Signal feed to the surface resonator is undertaken from the matchingcircuit 20 via shielded lines 21 and 22 which are connected to thoseends of the winding 19b respectively connected to the capacitors 19e and19f. The matching circuit 20 is thus directly galvanically coupled tothe winding 19b, with coupling to the winding 19a ensuing inductively.The structure shown in FIG. 2 is electrically capable of resonance, withthe resonant frequency determined by the windings 19a and 19b as well asby the capacitances of the capacitors 19c, 19d, 19e and 19f.

Good electrical symmetry is achieved by this type of coupling, and thusthe transmission field and the reception field are homogeneous.Transmission losses are avoided by the galvanic coupling. Moreover,because the structure disclosed herein does not require the use ofcoupling coils as in known surface resonators, there are no imagedisturbances resulting from the presence of such coupling coils, asresult from such conventional devices.

A convenient structure for mounting the windings 19a and 19b is shown inFIG. 3, wherein the windings are disposed on opposite sides of a ring 24having a rectangular cross section. As can be seen in FIG. 4, thisarrangement permits a further set of windings 19g and 19h (not shown inFIG. 3) to be mounted on the remaining opposite sides of the ring 24.The free ends of the winding 19g are connected to the matching circuit20 via capacitors 19i and 19j, and the free ends of the winding 19h areconnected to the matching circuit 20 via capacitors 19k and 19l, asdescribed above for the windings 19a and 19b. The windings 19g and 19hare thus connected to grounded potential, and one of those windings hasa direct galvanic connection to the signal terminal of the matchingcircuit 20, as also described above for windings 19a and 19b.

The windings 19g and 19h form a second resonator which, by differentdimensioning of the associated capacitors, can be set to a resonantfrequency different from the resonant frequency of the resonator formedby the windings 19a and 19b. Investigation of nuclei having differentresonant frequencies is thus possible in the embodiment of FIG. 4. Thewindings 19a, 19b, 19g and 19h may each consist of copper foil glued orotherwise attached to the ring 24.

An embodiment wherein two surface resonators 19 and 19', eachconstructed in accordance with the principles of the present invention,in a Helmholtz arrangement is shown in FIG. 5. The surface resonators 19and 19' are displaceably mounted on a stand 26 by releasable clamps 27and 28.

A homogeneous field in which the examination subject can be disposed isgenerated between the two surface resonators 19 and 19'.

The Helmholtz arrangement of two resonators 19 and 19' shown in FIG. 5has a number of advantages in comparison to a Helmholtz arrangementconsisting simply of surface coils. The LC relationship of the surfaceresonator disclosed herein is more favorable than that for surfacecoils, because the indictance L becomes smaller as a result of thecapacitance C of the capacitors 19c, 19d, 19e and 19f. A higher resonantcircuit quality and a better signal-to-noise ration are obtained as aresult of the more favorable LC ratio. The arrangement of the copperfoils permits the field distribution to be better controlled than for asurface coil consisting of wire. As a result of the aforementionedsymmetry obtained with the surface resonator as described above,substantially no "envelope" waves occur, i.e., currents arising on thereference conductor 23 of FIG. 2 due to asymmetry.

As in the case of the loop-gap resonators described in theaforementioned Magnetic Resonance In Medicine article, two resonantconditions having different resonant frequencies would also be possiblein the surface resonator disclosed herein if the two windings 19a and19b (or 19g and 19h) were not connected to each other. In a firstresonant condition wherein the magnetic field lines uniformly extendthrough both windings, the currents flow in the same direction in bothwindings. In a second resonant condition wherein the magnetic fieldlines close around a winding, the currents in the two windings flow inopposite directions. If the two windings 19a and 19b (or 19g and 19h)are cross-connected to each other, the first resonant condition issuppressed, so that homogeneous magnetic fields which uniformly extendthrough both windings do not lead to a resulting induction. Such asurface resonator thus no longer reacts to excitation fields which, forexample, derive from a body resonator, but only react to signals fromthe examination subject, whose field is always non-uniform. A decouplingof the surface resonator from the body resonator is thus achieved.

Although modifications and changes may be suggested by those skilled inthe art it is the intention of the inventor to embody within the patentwarranted hereon all changes and modifications as reasonably andproperly come within the scope of his contribution to the art.

I claim as my invention:
 1. A surface resonator arrangement for use innuclear magnetic resonance examination apparatus having means forsupplying signals to and receiving signals from said surface resonatorarrangement, said surface resonator arrangement including a surfaceresonator comprising:first and second windings each having free ends; afirst pair of series-connected capacitors connected across the free endsof said first winding, said first pair of capacitors having a junctiontherebetween disposed at an electrical symmetry point for said firstwinding; a second pair of series-connected capacitors connected acrossthe free ends of said second winding, said second pair of capacitorshaving a junction therebetween disposed at an electrical symmetry pointof said second winding; each of said first and second windings connectedto ground at said junctions; and means directly electrically connectingone of said windings to said means for supplying and receiving signals.2. A surface resonator arrangement as claimed in claim 1, wherein saidfirst and second windings each consist of a ribbon of electricallyconductive foil.
 3. A surface resonator arrangement as claimed in claim2, wherein said foil consists of copper.
 4. A surface resonatorarrangement as claimed in claim 1, further comprising a carrier for saidfirst and second windings consisting of a circular bar having arectangular cross section with pairs of opposite surfaces, said firstand second windings being respectively attached to opposite surfaces ofone of said pairs.
 5. A surface resonator arrangement as claimed inclaim 4, further comprising:third and fourth windings each having freeends; a third pair of series-connected capacitors electrically connectedacross the free ends of said third winding, said third pair ofcapacitors having a junction therebetween disposed at an electricalsymmetry point for said third winding; a fourth pair of series-connectedcapacitors electrically connected across the free ends of said fourthwinding, said fourth pair of capacitors having a junction therebetweendisposed at an electrical symmetry point of said fourth winding; saidthird and fourth windings being connected to ground at said junctions;and means for directly electrically connecting one of said third orfourth windings to said means for supplying and receiving signals,saidthird and fourth windings and said third and fourth pairs of capacitorsforming a further surface resonator, said third and fourth pairs ofcapacitors having in combination a capacitance different from thecapacitance of said first and second pairs of capacitors in combinationso that said further surface resonator has a different resonantfrequency from said surface resonator.
 6. A surface resonatorarrangement as claimed in claim 1, further comprising an additionalsurface resonator constructed identically to said surface resonator,each of said surface resonator and said additional surface resonatorhaving a winding axis, and means for mounting said surface resonator andsaid additional surface resonator in a Helmholtz arrangement with thewinding axes of said surface resonator and said additional surfaceresonator in a line.
 7. A surface resonator arrangement for use in anuclear magnetic resonance examination apparatus having means forsupplying signals to and receiving signals from said surface resonatorarrangement, said surface resonator arrangement comprising:a circularcarrier having a rectangular cross section, said carrier having firstand second pairs of opposite surfaces; a first surface resonator havingfirst and second windings respectively disposed on said first pair ofopposite surfaces of said carrier, each of said windings having freeends and said first surface resonator further having first and secondcapacitor means for respectively connecting said free ends of saidwindings, each of said first and second capacitor means having anelectrical symmetry point for the winding connected thereto, each ofsaid symmetry points being connected to ground; a second surfaceresonator having windings respectively disposed on said second pair ofopposite surfaces of said carrier, said windings each having free endsand said second surface resonator further having third and fourthcapacitor means for respectively connecting the free ends of each ofsaid windings in said second surface resonator, each of said third andfourth capacitor means having an electrical symmetry point for thewinding connected thereto, said electrical symmetry points beingconnected to ground; means for directly electrically connecting one ofthe windings of said first surface resonator to said means for supplyingand receiving; and means for directly connecting one of said windings ofsaid second surface resonator to said means for supplying andreceiving,said first and second capacitor means in combination having acapacitance different from the capacitance of said third and fourthcapacitor means so that said first surface resonator has a resonantfrequency different from the resonant frequency of said second surfaceresonator.
 8. A surface resonator arrangement as claimed in claim 7,wherein each of the windings of said first and second surface resonatorsconsists of a ribbon of electrically conductive foil.
 9. A surfaceresonator arrangement as claimed in claim 8, wherein said foil consistsof copper.